Field of the Invention
This invention relates to defect detection performed by irradiating samples with terahertz light. The invention in particular relates to elimination of interference patterns caused by terahertz light. The “terahertz light” as used herein is assumed to contain electromagnetic radiation particularly with a frequency range of 100 GHz to 10 THz.
Description of Related Art
Unlike X-rays, terahertz radiation is non-ionizing electromagnetic radiation which is safe to human health, and yet is very transmissive to various materials such as paper, plastics, semiconductors, or the like. Therefore, great expectations are placed on terahertz radiation as effective radiation for detecting defects that cannot be detected with X-rays, visible light, infrared light or the like.
While it is quite difficult to find a terahertz imaging device which is able to acquire images in real time, an imaging device which images a sample using a terahertz light source and a terahertz camera is disclosed, for example, in B. N. Behnken et al., Proc. SPIE Vol. 6893 (2008) p 68930L, FIG. 6 (hereafter, referred to as Non-Patent Document 1), and A. W. M. Lee et al., IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 18 (2006) p 1415 (hereafter, referred to as Non-Patent Document 2).
Non-Patent Document 1 describes an experimental arrangement as shown in FIG. 9. Referring to FIG. 9, a quantum cascade laser 100 is attached to a cryostat 101 to be cooled to about 10 K and emits 3.6 THz monochromatic waves. The quantum cascade laser has a peak power of about 5 mW and is driven with a duty cycle of about 20%. Thus, the quantum cascade laser has a time-average power of about 1 mW. Monochromatic light from the quantum cascade laser is transformed into collimated beams by a first off-axis paraboloidal minor 102 (F/1, with focal length of 50.8 mm) and applied to a sample 103 placed on a sample plane. The terahertz radiation passing through the sample is focused into an image on an infrared light detecting microbolometer array sensor 106 with 160×120 pixels (frame rate of 30 Hz) mounted on a camera 105, by a second off-axis paraboloidal mirror 104 (F/2, with focal length of 101.6 mm).
A terahertz image 107 obtained by the aforementioned arrangement is shown in FIG. 10. In Non-Patent Document 1, a steel blade covered with two layers of vinyl tape is used as the sample. Whereas Non-Patent Document 1 says that the steel blade is clearly recognized in the image, it can be seen from FIG. 10 that concentric interference patterns 108 have appeared together with the image of the blade. These interference patterns 108 are generated due to high coherency of the quantum cascade laser.
Non-Patent Document 2 describes an experimental arrangement as shown in FIG. 11. A quantum cascade laser 200 is attached to a cryostat 201 to be cooled to about 33 K and emits 4.3 THz monochromatic waves. The quantum cascade laser has a peak power of about 50 mW and is driven by a duty cycle of 25%. Thus, the quantum cascade laser has a time-average power of about 12.5 mW. Monochromatic light from the quantum cascade laser is transformed by a first off-axis paraboloidal minor 202 (F/1, with focal length of 50 mm) into collimated beams, which are collected by a second off-axis paraboloidal mirror 203 (F/2, focal length 100 mm) and applied to an envelope (sample) 204. An image of the envelope is formed on an infrared light detecting microbolometer array sensor 207 with 320×240 pixels in a camera 206 through a silicon lens 205 (F/1, with focal length of 25 mm). A terahertz image 208 obtained by this arrangement is shown in FIG. 12. Characters 209 of “MIT” written on the envelope with a pencil can be recognized clearly.
As seen from FIG. 10, the image obtained by the apparatus described in Non-Patent Document 1 includes not only the steel blade to be detected but also concentric interference patterns 108 generated due to high coherency of the quantum cascade laser. The presence of the interference patterns 108 makes it difficult to view the object to be detected.
Whereas there exist light sources with low coherency, luminance of this type of light source is not so high as that of a quantum cascade laser. For this reason, when the same inspection is conducted using a light source with a low coherency, a high signal-to-noise ratio cannot be obtained, and thus it is difficult to apply the light source to a field of non-destructive inspection of defects in a material or the like.
A backward-wave tube is one of monochromatic light sources with high coherency and high luminance. Interference patterns are generated also in a backward-wave tube. In order to reduce the interference patterns, for example, the tube voltage may be changed at an operating point of 1 kV by about 50 V, but it is impossible to eliminate the interference patterns completely.
It is also conceivable to use a spatial filter in image processing for eliminating such interference patterns. However, when defects of a material are to be detected, the spatial filter may delete the defects. Therefore, a better method is desired.
No interference patterns are recognized in FIG. 12 principally because in the arrangement shown in FIG. 11, the beams are likely focused at a focal position of the second off-axis paraboloidal mirror 203 in order to extract only main beams of the beam patterns from the quantum cascade laser, and the interference patterns are made thinner by diffusion and scattering by the paper envelope as the sample. Even if aperture is set, diffraction patterns due to aperture edge likely appear in a long-wavelength region such as terahertz radiation. Thus, the method described in Non-Patent Document 2 is believed to be greatly affected by diffusion and scattering caused by paper.
As described above, currently available terahertz light sources with high luminance have high coherency. Therefore, when terahertz imaging technology is applied to a field of non-destructive inspection of defects in materials or the like, it will face a problem of presence of interference patterns.
This invention has been made in view of such circumstances, and an object of the invention is that, in inspection performed by irradiating a sample with beams from a terahertz light source and capturing an image of the sample with a terahertz camera, interference patterns caused by terahertz light are eliminated by the image so as to facilitate visual inspection of an object to be inspected.